fix(metrics): use client-side filtering for --filter flag, add --tag-filter

[platinummonkey]

Summary

Fixes #53 - pup metrics list --filter now correctly filters metrics by name patterns instead of returning empty results.

The --filter flag was incorrectly using the Datadog API's tag_filter parameter, which filters by Datadog tags (like env:prod), not metric names. This caused all metric name filters to return empty results.

Changes

• Add matchMetricName() helper (cmd/metrics.go:637) - Implements wildcard pattern matching supporting * (match any characters) and ? (match single character)
• Apply client-side filtering (cmd/metrics.go:748-755) - Filters metric names after fetching from API when --filter is used
• Add --tag-filter flag (cmd/metrics.go:484) - Exposes actual tag filtering functionality for users who want to filter by Datadog tags
• Update documentation (cmd/metrics.go:162-208) - Clarifies the difference between --filter (name patterns) and --tag-filter (Datadog tags)
• Comprehensive tests (cmd/metrics_test.go:205-468) - 30+ test cases covering wildcard patterns, edge cases, and real-world examples

Testing

Unit tests:

go test ./cmd -run TestMatchMetricName  # 30+ pattern matching tests
go test ./cmd -run TestRunMetricsList   # Integration tests

Manual testing with real API:

# Before fix: returns empty
pup metrics list --filter="system.cpu"
# Output: {"from": "...", "metrics": []}

# After fix: returns matching metrics
pup metrics list --filter="system.cpu*"
# Output: {"from": "...", "metrics": ["system.cpu.user", "system.cpu.system", ...]}

# Wildcard patterns work as documented
pup metrics list --filter="*.cpu.*"
pup metrics list --filter="*request*"

# New --tag-filter flag for tag-based filtering
pup metrics list --tag-filter="env:prod"
pup metrics list --filter="system.*" --tag-filter="env:prod"  # Combine both

Test coverage:

• cmd package: 24.8% (maintained)
• pkg packages: >80% (maintained)
• All tests pass with race detection

Examples

# Filter by metric name patterns (client-side)
pup metrics list --filter="system.*"          # All system metrics
pup metrics list --filter="*.cpu.*"           # All CPU metrics
pup metrics list --filter="*request*"         # Metrics containing "request"
pup metrics list --filter="kafka_index*"      # Kafka index metrics

# Filter by Datadog tags (server-side)
pup metrics list --tag-filter="env:prod"                    # Production metrics
pup metrics list --tag-filter="env:prod,service:api"        # Multiple tags (AND)

# Combine both filters
pup metrics list --filter="system.*" --tag-filter="env:prod"  # System metrics in prod

Breaking Changes

None. The --filter flag behavior is fixed to match its documentation. Since it was broken (always returned empty), no existing workflows should break.

Related Issues

Closes #53

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🤖 Generated with Claude Code

[github-actions]

📊 Test Coverage Report

Overall Coverage: 81.7%

Threshold: 80% ✅

Coverage by Package

## Coverage by Package

- github.com/DataDog/pup/pkg/auth/callback/server.go:40: 81.2%
- github.com/DataDog/pup/pkg/auth/dcr/client.go:28: 100.0%
- github.com/DataDog/pup/pkg/auth/dcr/types.go:24: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/client.go:22: 100.0%
- github.com/DataDog/pup/pkg/auth/oauth/pkce.go:24: 85.7%
- github.com/DataDog/pup/pkg/auth/storage/factory.go:53: 94.7%
- github.com/DataDog/pup/pkg/auth/storage/keychain.go:44: 42.9%
- github.com/DataDog/pup/pkg/auth/storage/storage.go:58: 71.4%
- github.com/DataDog/pup/pkg/auth/types/types.go:23: 100.0%
- github.com/DataDog/pup/pkg/client/client.go:32: 94.4%
- github.com/DataDog/pup/pkg/config/alias.go:26: 100.0%
- github.com/DataDog/pup/pkg/config/config.go:22: 100.0%
- github.com/DataDog/pup/pkg/formatter/formatter.go:31: 100.0%
- github.com/DataDog/pup/pkg/useragent/useragent.go:32: 100.0%
- github.com/DataDog/pup/pkg/util/time.go:28: 96.0%

## Summary

total:								(statements)		81.7%

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📈 Coverage Status: ✅ PASSED - Coverage meets minimum threshold

_{Updated for commit 392f1c3}